Byproducts & Nuclear Waste

A running reactor doesn't just make heat. Fission splits atoms into a zoo of unstable daughter isotopes, and breeding stacks its own poisons on top. All of it accrues inside the core while it runs, and you watch it climb live in the Fuel Port.

Almost none of it is worth keeping. When you withdraw any byproduct it consolidates into one generic Nuclear Waste item — a mixed disposal vial, not a shelf of collectible isotopes. This page is the byproduct model: what accrues, what you can pull, and what you actually get when you pull it.

The byproduct smorgasbord

While a core is critical it stages seven distinct byproduct species. You see each one live and named in the Fuel Port's internals view, with its amount ticking up as the core burns:

The five fission products accrue steadily any time the core is splitting fuel — a cruising core stages its first whole unit of a major product (Cs-137) in about a minute. They carry no effect on the sim; they are pure disposal load.

Pu-240 is the over-breed poison. Bred Pu-239 left in a breeder too long walks to Pu-240, so its pile climbing is the signal you missed a harvest window. See Breeding for the harvest-timing skill that keeps it down.

Xe-135 is a live reactivity poison the sim actually reads — the source of xenon dead-time transients — and it's covered as its own lever below.

These byproducts stage as fractional amounts internally; you can only withdraw whole units, and the fractional remainder stays staged (never lost) until it rounds up.

Consolidation to Nuclear Waste

Here is the important part: those seven species are things you see, not things you keep.

When you click any staged byproduct in the Fuel Port to withdraw it, the whole units you pull are handed to you as a single generic Nuclear Waste item — a mixed vial of "cesium, strontium, technetium, iodine and the rest, consolidated for disposal." Cs-137, Sr-90, Tc-99, I-131, Mo-99, the over-bred Pu-240, and vented Xe-135 all fold into that one item on collection.

So all seven produced isotopes are unobtainable as distinct items. The core produces them, you monitor them by name, but the moment they leave the reactor they lose their identity. Nuclear Waste is terminal: radioactive, non-recyclable, and its downstream use is still to be determined ("store, dump, or reprocess — a use comes later"). Don't stockpile it expecting a payoff yet.

The one thing that does not consolidate is the bred fissile itself. Pu-239 you harvest before it degrades withdraws as its own loadable Plutonium-239 fuel item, not as waste — because it's the prize, not the trash. Getting waste when you meant to get fuel means you harvested too late and it walked to Pu-240.

Xenon as a lever

Xe-135 wears two hats, and this is the one byproduct worth thinking about strategically.

It is a poison you fight. The sim reads the zonal Xe-135 pool as a neutron poison that chokes the chain reaction, producing the dead-time transients after a throttle change (see Operation for how xenon shapes the cruise band). Left alone it builds, peaks, and burns off on its own schedule.

It is also a species you can withdraw. Venting Xe-135 by hand from the Fuel Port pulls whole units of the poison out of the core — and because the sim reads that pool as reactivity, removing it inserts positive reactivity. The core runs hotter the instant you vent. That makes venting a deliberately double-edged lever: it clears the poison-out dead-time and lets the chain climb, but it spikes core temperature at the same time. Vent into a redline and you've made a bad situation worse.

Vented xenon still consolidates to Nuclear Waste like everything else — you don't get a jar of Xe-135, you get a jar of waste and a hotter core.

Reserved isotopes

The reactor registry defines more isotopes than the core actually produces. Two of them are marked waste-exempt: if the sim ever staged them, they would withdraw as their own distinct items instead of folding into Nuclear Waste. But nothing produces them today, so they exist as lore only:

Several other isotopes (Np-239, Pu-241, I-135, Sm-149, Kr-85, and more) are registered as items to back the fuel chain and the internals display, but they aren't withdrawable byproducts either. The full registry is the Neutronics Roster; treat anything not in the seven-species table above as scenery, not loot.

See also

• Neutronics: the overview and where this fits
• Fuel & Isotopes: the mine → enrich → burn → breed chain that feeds the core
• Breeding: geometry-gated Pu-239 and the harvest window that keeps Pu-240 down
• Operation: throttle bands, xenon transients, and SCRAM
• Consoles & Fuel Port: the two surfaces, and where you watch byproducts accrue
• Neutronics Roster: every registered fuel, isotope, and machine